@testit-sdk/express

AbTest

AbTest is a lightweight TypeScript library for implementing A/B testing in Express.js applications. It provides an easy way to create and manage experiments, assign users to variants, and check variant assignments. This package is made to work with Test It but can be used without it if you have your own analytics system.

Features

• Simple API for creating and managing A/B tests
• Supports multiple experiments and variants
• Persistent variant assignment using cookies
• Weighted random selection of variants
• TypeScript support

Installation

npm install @testit-sdk/express

Usage

Basic Example

import AbTest from "@testit-sdk/express";
import express from "express";

const app = express();

app.get("/", (req, res) => {
  const abTest = new AbTest({
    experimentId: "66d6381d630d87321d2937c6",
    variants: [
      { id: "66d6621d630d87341d2937f3", name: "baseline", weight: 50 },
      { id: "66d6681c630d87341d2936d4", name: "v_1", weight: 50 },
    ],
    req,
    res,
  });
  if (abTest.isVariant("v_1")) {
    res.send('<button style="background-color: blue;">Click me!</button>');
  } else {
    res.send('<button style="background-color: red;">Click me!</button>');
  }
});

app.listen(3000, () => console.log("Server running on port 3000"));

API

Constructor

• options.experimentId: Unique identifier for the experiment
• options.variants: Array of variant objects with id, name, and weight properties
• options.req: Express request object
• options.res: Express response object

Methods

• isVariant(variantName: string): boolean: Check if the current user is assigned to a specific variant
• isV(variantName: string): boolean: Alias for isVariant

How It Works

AbTest uses cookies to persistently assign users to variants. When a user first encounters an experiment, they are randomly assigned to a variant based on the specified weights. This assignment is stored in a cookie and reused for subsequent requests.